US Vice President Joe Biden condemned Russia's formal annexation of the Crimea peninsula on Tuesday as a "blatant violation of international law."
Biden, speaking in Poland, said he planned to "reassure" regional allies Tuesday as Russia tightened its grip on the Ukraine's breakaway region of Crimea.
He called Russia's deployment of troops to the region a "land grab" and warning of further sanctions against Moscow.
Biden made his speech soon after Putin officially announced in a speech on Tuesday that Crimea would be officially integrated into the Russian Federation, after an overwhelming vote in favour of joining Russia by citizens of the peninsula.
Biden also said that Russia's economic and political isolation will increase if it persists in its stance on Ukraine.
"It will in fact see additional sanctions by the United States and the EU," Biden said during a visit to Warsaw, after the Kremlin announced it now considers Crimea a part of Russia.